This invention relates to a heating device, of the kind having a fuel container in which a liquid fuel, for example oil, is evaporated within an evaporation chamber, having a combustion chamber which is mounted on the fuel container and in which the evaporated fuel is burnt, having a heating tube which is connected to the combustion chamber, and having a gas feed-back or return device which air-tightly connects the heating tube to the evaporation chamber in the fuel container and returns into the evaporation chamber a portion of the combustion gases which pass into the heating tube.
In a known heating device of this kind of construction, which is operated for example as a so-called air-oil heater, the heating gases are produced by means of an operating flame on the fuel oil which is in the fuel container. However, this results in a sooty flame, which has disadvantageous consequences. Therefore the present invention is basically concerned with a heating device of this kind, but which operates without any such flame on the surface of the oil which is disposed in the fuel container, the fuel container thus serving as the evaporation tank. This however is only possible if particular care is taken to ensure that certain parts of the heating device, for example the cover of the fuel container, the fuel container itself and also the combustion chamber, are heat-insulated.
It has also been found that operating a heating device without the above-mentioned flame on the surface of the fuel oil in the fuel container is only possible when the above-mentioned gas feedback device is in the form of a tube bend and a gas feedback tube, of a special contruction. The tube bend advantageously comprises a cast material. In this kind of arrangement, the fuel oil which is vaporised in the evaporation tank is heated by the heating gases which are fed back from the heating tube by way of the gas feedback device, so as to provide constant operation of the heating device without the above-mentioned operating fire on the fuel oil surface. With this gas feedback device, the straight gas feedback tube is fixedly connected at its lower end to the upper cover of the fuel container, for example by means of a flange connection, while the upper end of the gas feedback tube is connected to the tube bend by way of a plug-in connection. In this arrangement, in which the tube bend is for example a 180.degree. bend, one half of the tube bend is arranged within the heating tube, while the open end of said half, being disposed centrally within the heating tube, faces downwardly in the direction of the combustion chamber. The other half of the tube bend is arranged outside the heating tube and is there connected to the gas feedback tube. Now, in such a heating device, the oil combustion gases which rise out of the heat-insulated evaporation tank are burnt in the combustion chamber, which has perforations, and are then in part returned to the evaporation tank, due to the reduced pressure which prevails in the evaporation tank, passing by way of the portion of the tube bend which is within the heating tube, the outer portion of the tube bend, and the gas feedback tube. This arrangement can possibly result in secondary combustion. It is therefore particularly important for such a heating device that the heating gases which are fed back to the evaporation tank are returned thereto at a high temperature, without substantial cooling of the gases occurring over the feed-back path. Such cooling can occur particularly when the heating device is installed in the open air, as so-called point heaters.